The change in pressure and velocity of a supersonic flow of air as it passes over a surface, which drops away from the flow. As the surface drops away, the air tries to follow it. In changing its direction, the air speeds up to a higher supersonic velocity, and its static pressure decreases.

There is no change in the total amount of energy as the air passes through an expansion wave, and so there is no noise as there is when air passes through a shock wave.

“Expect (altitude) at (time) or (fix)” (air traffic control).

A portion of an instrument departure clearance, to provide a pilot with an altitude to be used in the event of two-way radio communication failure.

“Expect departure clearance (time)” (air traffic control). The time an operator can expect a gate release. This is used in the Fuel Advisory Departure (FAD) program.

“Expect further clearance (time)” (air traffic control). The time a pilot can expect to receive clearance beyond a clearance limit.

“Expect further clearances via (airways, routes, or fixes)” (air traffic control). This phrase is used by air traffic controllers to inform a pilot of the routing he may expect if any part of the route beyond a short range clearance limit differs from that filed.

“Expedite” (air traffic control). A warning used by air traffic controllers when prompt compliance is required in order to avoid the development of an imminent situation.